OKLAHOMA CITY − A man and woman sleeping outside in Oklahoma were deliberately set on fire Tuesday morning and at least one was critically burned, according to police.

Oklahoma City Police received a call around 6 a.m. reporting two people had been set on fire. Police believe an assailant lit them on fire and then fled the scene.

The victims are at a local burn center where the woman is in critical condition. The man is expected to survive.

Oklahoma City police arrested a 70-year-old suspect in the case, also homeless.

Shelah Farley, the clinical director for the Mental Health Association of Oklahoma, said people forced to live outside are extremely vulnerable to violence.

"There's no place for them to be inside where it's safe," Farley said. "They have no where to lock up their belongings or just be inside somewhere away from crime. They're always out in the open."

Between 14% and 21% of the homeless population have been victims of violence compared to 2% of the general population, according to research published in the journal Violence and Victims.

Farley said this heightened risk of victimization forces people experiencing homelessness to be in a constant survival mode, which alters their brain chemistry that can lead to mental illnesses.

She said she often sees unhoused people with depression and hopelessness due to their situation.

"They're looked down upon. They're talked bad about. People talk to them poorly, almost like they're like scum," Farley said. "Over time, you begin to believe that about yourself, if that's what everyone you encounter continues to reinforce."

The stigma can make them targets, too she said.

"At the end of the day it's really disheartening, and it's sickening to know that another human being would think that way about another human being just because of their situation," Farley said.

Cristi Colbert, an Oklahoma City resident, told USA TODAY she became sick to her stomach after learning of the assault. In 2016, Colbert, 56, became homeless for the first time and said she bounced between sleeping on a friend's couch, inside her car, or sometimes, outside at a park.

“When you get ready to sleep, you hope and you pray that you picked the right dumpster to sleep behind, that it’s the safest," said Colbert, who is no longer homeless. "But there are no guarantees − you have to sleep with one eye open.”

The attack comes a month after the Supreme Court ruled cities and states can enforce laws prohibiting people from sleeping on public property, a decision advocates worry will only make the crisis worse, forcing homeless into the criminal system, making getting housing even more difficult.

Colbert said people should seek safety indoors if they don't have housing.

“The whole thing is horrific," she said. "People deserve a good safe place to live, everyone deserves to have a home."